It is known that an antimony-containing metal oxide catalyst, more particularly, a catalyst composed of antimony oxide and an oxide of at least one kind of metal (simply referred to as a polyvalent metal hereinafter) selected from the group consisting of iron, cobalt, nickel, manganese, uranium, tin, and copper is useful for the production of unsaturated aldehydes through oxidation of olefins, the production of diolefins through oxidation dehydrogenation of olefins, and the production of unsaturated nitriles through ammoxidation of olefins. For instance, known catalysts are composed of antimony oxide and an oxide or iron, cobalt, or nickel, which is claimed as a catalyst to be useful for the production of acrylonitrile through ammoxidation of propylene in Japanese Patent Publication No. 19111/63. A catalyst composed of antimony oxide and iron oxide is also disclosed in U.S. Pat. No. 3,197,419. Another catalyst composed of antimony oxide and tin oxide is disclosed in U.S. Pat. No. 3,152,170, a catalyst composed of antimony oxide and uranium oxide is disclosed in U.S. Pat. No. 3,308,151, and a catalyst composed of antimony oxide and manganese or copper oxide is disclosed in U.S. Pat. Nos. 3,200,081 and 3,340,291.
However, these catalysts do not produce satisfactory yields of the intended product, and efforts have been made to improve their performance by adding other elements. For instance, there is disclosed in U.S. Pat. No. 3,668,147 a multiple accelerated antimony-polyvalent metal oxide catalyst in which a catalyst composed of antimony oxide and iron oxide, antimony oxide and tin oxide, or antimony oxide and uranium oxide, is incorporated with tellurium oxide and an oxide of at least one element selected from the group consisting of vanadium, molybdenum, and tungsten.
These antimony-containing metal oxide catalysts are prepared from compounds which become oxides after decomposition with heating. Usually, the raw materials for respective components are mixed and precipitated, and the precipitates are dried and calcined. However, this method does not invariably provide catalysts having preferred activity and properties because the activity and properties of the catalysts are greatly affected even by slight variation of the calcination temperature.
A solution to this problem was disclosed in U.S. Pat. No. 4,049,575. According to this disclosure, an antimony-containing metal oxide catalyst is prepared by calcining an antimony-polyvalent metal oxide composition that constitutes the catalyst matrix, impregnating the calcined composition with an aqueous solution containing tellurium and at least one element selected from the group consisting of molybdenum, vanadium, and tungsten, and drying and calcining the resulting composition. In preparation of catalysts by the impregnation method, it is very important to prepare a homogeneous, stable impregnant containing catalyst components in predetermined quantities. In this connection, the method disclosed in U.S. Pat. No. 4,049,575 involves industrial problems in the preparation of the impregnant containing tellurium and other additives.
In other words, the above-mentioned impregnating solution is prepared by dissolving metallic tellurium or a tellurium compound such as tellurium dioxide and tellurous acid in nitric acid, and then mixing the tellurium-containing nitric acid solution with a solution containing other components. This method is industrially feasible from the standpoint of operation and economy. In addition, this method is advantageous in that the catalyst components other than tellurium are comparatively high in solubility in the form of nitrate and they can be prepared in pure form with ease. Nevertheless, this method is not desirable because the tellurium-containing nitric acid solution tends to form a precipitate on addition of an aqueous solution of a water-soluble molybdenum compound such as ammonium metamolybdate and ammonium paramolybdate, or on addition of an aqueous solution of a water-soluble tungsten compound such as ammonium metatungstate and ammonium paratungstate, during the preparation of an impregnant containing both a tellurium component and a molybdenum component or tungsten component. Thus it has been impossible to prepare homogeneous, stable solutions in a broad range of concentrations. This is true particularly in the preparation of a tellurium-containing nitric acid solution which contains a tungsten component alone or a tungsten component and molybdenum component together. As mentioned above, it has been very difficult to prepare, by using a tellurium-containing nitric acid solution, a homogeneous, stable impregnant containing tellurium and molybdenum and/or tungsten.
Such an impregnant may be dispensable in a process in which a nitric acid solution of tellurium and an aqueous solution of ammonium molybdate or ammonium tungstate are used independently. According to such a process, the catalyst is prepared by impregnation in two steps. That is, the catalyst matrix is impregnated with a nitric acid solution of tellurium, followed by drying and calcining. The resulting composition is then impregnated with an aqueous solution of ammonium paratungstate, followed by drying and calcining. The steps may be inverted. A detail description is given later in Comparative Example 1-b. This process, however, requires complicated steps, and is not industrially feasible.
According to the process disclosed in U.S. Pat. No. 4,049,575, the homogeneous, stable impregnating solution is prepared by dissolving telluric acid and a water-soluble molybdenum compound in water. Telluric acid, however, is disadvantageous as a raw material for industrial catalysts because it is expensive due to the fact that it is produced and purified by many steps including oxidizing metallic tellurium with chloric acid or oxidizing tellurium dioxide with potassium permanganate. Furthermore, it has no broad area of industrial application. In addition, telluric acid of high purity is not readily available, presumably due to the nature of its production process.
U.S. Pat. No. 3,474,042 discloses a process for preparing a catalyst by impregnating a carrier such as silica and alumina with a homogeneous solution containing a tellurium component and a molybdenum or tungsten component. According to this disclosure, the tellurium component is prepared from telluric acid, the molybdenum component is prepared from a peroxymolybdenum compound obtained through the reaction of ammonium molybdate and hydrogen peroxide, and the tungsten component is prepared from a peroxytungsten compound obtained through the reaction of ammonium tungstate and hydrogen peroxide. This method is still industrially infeasible because expensive tellurium acid is used as a raw material of the tellurium component and safety problems are involved in the use of a large quantity of hydrogen peroxide for the preparation of a peroxymolybdenum compound and peroxytungsten compound.
In order to solve the aforesaid problems involved in preparing the impregnating solution containing tellurium and other components by using a nitric acid solution of tellurium, the present inventors carried out a series of researches which led to this invention. This invention relates to an improvement of U.S. Pat. No. 4,049,575 cited above.